Solid-state imaging device

ABSTRACT

A solid-state imaging device includes: a first electrode formed above a semiconductor substrate; a photoelectric conversion film formed on the first electrode and for converting light into signal charges; a second electrode formed on the photoelectric conversion film; a charge accumulation region electrically connected to the first electrode and for accumulating the signal charges converted from the light by the photoelectric conversion film; a reset gate electrode for resetting the charge accumulation region; an amplification transistor for amplifying the signal charges accumulated in the charge accumulation region; and a contact plug in direct contact with the charge accumulation region, comprising a semiconductor material, and for electrically connecting to each other the first electrode and the charge accumulation region.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a continuation application of PCT International Application No.PCT/JP2012/003710 filed on Jun. 6, 2012, designating the United Statesof America, which is based on and claims priority of Japanese PatentApplication No. 2011-139944 filed on Jun. 23, 2011. The entiredisclosures of the above-identified applications, including thespecifications, drawings and claims are incorporated herein by referencein their entirety.

FIELD

The present disclosure relates to a solid-state imaging device includinga plurality of pixels which are arranged in a matrix and each of whichincludes a photoelectric conversion unit.

BACKGROUND

In recent years, MOS (Metal Oxide Semiconductor) solid-state imagingdevices are mounted in mobile cameras, in-vehicle cameras, andmonitoring cameras.

These solid-state imaging devices are required to capture images at highresolution, and miniaturization and higher pixel counts are necessaryfor the solid-state imaging devices. In conventional solid-state imagingdevices, the size of a photodiode is also reduced due to a finer pixel.This has caused a problem that the saturation signal level is reduced bydecreasing the volume of a photodiode, and sensitivity is reduced bydecreasing the aperture ratio.

As a solid-state imaging device for solving such a problem, a layeredsolid-state imaging device is proposed. The layered solid-state imagingdevice has a photoelectric conversion film stacked on the top surface ofa semiconductor substrate. Furthermore, light is incident from above thelayered films. The solid-state imaging device has a structure in whichcharges generated in the photoelectric conversion film by photoelectricconversion are read out using a Charge Coupled Device (CCD) circuit or aComplementary MOS (CMOS) circuit.

Patent Literature (PTL) 1 discloses a conventional layered solid-stateimaging device. FIG. 13 illustrates a circuit diagram of a pixel circuitin the solid-state imaging device described in PTL 1. The pixel circuitshown in FIG. 13 has a charge accumulation region (FD) and a pixelelectrode 15 a electrically connected to each other, and the voltagevaries depending on the incident light intensity. Furthermore, thecharge accumulation region is electrically connected to a gate electrodeof an amplification transistor 17 b. With this, the pixel circuit iscapable of amplifying an amount of voltage change in the chargeaccumulation region to read out the resulting voltage to a signal line17 d.

In the foregoing layered solid-state imaging device, although thephotoelectric conversion film is stacked on the top of a wiring layerused in a reading out circuit and a signal processing circuit, thecharges generated by photoelectric conversion are accumulated in thecharge accumulation region provided in the semiconductor substrate. Thisis why the charges generated by photoelectric conversion are transmittedto the charge accumulation region via metal plugs.

CITATION LIST Patent Literature

-   [PTL 1] Japanese Patent No.4444371

SUMMARY Technical Problem

However, the conventional technique has a problem of increasing noisebecause crystal defects increase at the contact interface between acharge accumulation region and a metal plug due to a silicon alloyingreaction.

In view of this, the present disclosure provides a solid-state imagingdevice which reduces noise by suppressing crystal defects caused by analloying reaction.

Solution to Problem

In order to solve such a problem, a solid-state imaging device accordingto one embodiment of the present disclosure is a solid-state imagingdevice including a plurality of pixels arranged in a matrix, thesolid-state imaging device including: a semiconductor substrate; a firstelectrode formed above the semiconductor substrate for each of thepixels, and electrically isolated from an adjacent one of the pixels; aphotoelectric conversion film formed on the first electrode, thephotoelectric conversion film converting light into signal charges; asecond electrode formed on the photoelectric conversion film; a chargeaccumulation region formed in the semiconductor substrate for each ofthe pixels, and electrically connected to the first electrode in thepixel, the charge accumulation region accumulating the signal chargesconverted from the light by the photoelectric conversion film; a resetgate electrode formed for each of the pixels, the reset gate electroderesetting the charge accumulation region; an amplification transistorformed for each of the pixels, the amplification transistor amplifyingthe signal charges accumulated in the charge accumulation region in thepixel; and a contact plug formed, for each of the pixels, in directcontact with the charge accumulation region and comprising asemiconductor material, the contact plug being for electricallyconnecting to each other the first electrode and the charge accumulationregion in the pixel.

With this, in the solid-state imaging device according to one embodimentof the present disclosure, a semiconductor material is used for thecontact plug for electrically connecting to each other the photoelectricconversion unit and the charge accumulation region which are essentialsin a layered solid-state imaging device. As a result, an alloyingreaction does not occur at the contact interface between the chargeaccumulation region and the contact plug. Thus, the solid-state imagingdevice can reduce the crystal defects generated in a contact portionbetween the charge accumulation region and the contact plug, therebydecreasing noise.

Advantageous Effects

As described above, the present disclosure provides a solid-stateimaging device which can reduce noise by suppressing crystal defectscaused by an alloying reaction.

BRIEF DESCRIPTION OF DRAWINGS

These and other objects, advantages and features of the disclosure willbecome apparent from the following description thereof taken inconjunction with the accompanying drawings that illustrate a specificembodiment of the present invention.

FIG. 1 illustrates a circuit diagram showing a solid-state imagingdevice according to an embodiment 1 of the present disclosure.

FIG. 2 illustrates a timing chart showing an operation of thesolid-state imaging device according to the embodiment 1 of the presentdisclosure.

FIG. 3 illustrates a cross-sectional view of the solid-state imagingdevice according to the embodiment 1 of the present disclosure.

FIG. 4 illustrates a graph showing electric field intensity as afunction of an N-layer concentration in a P-N junction, according to theembodiment 1 of the present disclosure.

FIG. 5A illustrates a schematic view showing a band structure under adark condition along a line through a contact plug and a chargeaccumulation region, for the solid-state imaging device according to theembodiment 1 of the present disclosure.

FIG. 5B illustrates a schematic view showing a charge profile under thedark condition along the line through the contact plug and the chargeaccumulation region, for the solid-state imaging device according to theembodiment 1 of the present disclosure.

FIG. 5C illustrates a schematic view showing a band structure under anillumination condition along the line through the contact plug and thecharge accumulation region, for the solid-state imaging device accordingto the embodiment 1 of the present disclosure.

FIG. 5D illustrates a schematic view showing a charge profile under theillumination condition along the line through the contact plug and thecharge accumulation region, for the solid-state imaging device accordingto the embodiment 1 of the present disclosure.

FIG. 6 illustrates a cross-sectional view of the solid-state imagingdevice according to an embodiment 2 of the present disclosure.

FIG. 7 illustrates a circuit diagram of a pixel circuit according to theembodiment 2 of the present disclosure.

FIG. 8 illustrates a cross-sectional view of the solid-state imagingdevice according to an embodiment 3 of the present disclosure.

FIG. 9A illustrates a cross-sectional view showing a step inmanufacturing the solid-state imaging device according to the embodiment1 of the present disclosure.

FIG. 9B illustrates a cross-sectional view showing a step inmanufacturing the solid-state imaging device according to the embodiment1 of the present disclosure.

FIG. 9C illustrates a cross-sectional view showing a step inmanufacturing the solid-state imaging device according to the embodiment1 of the present disclosure.

FIG. 9D illustrates a cross-sectional view showing a step inmanufacturing the solid-state imaging device according to the embodiment1 of the present disclosure.

FIG. 9E illustrates a cross-sectional view showing a step inmanufacturing the solid-state imaging device according to the embodiment1 of the present disclosure.

FIG. 10A illustrates a cross-sectional view showing a step inmanufacturing the solid-state imaging device according to the embodiment2 of the present disclosure.

FIG. 10B illustrates a cross-sectional view showing a step inmanufacturing the solid-state imaging device according to the embodiment2 of the present disclosure.

FIG. 10C illustrates a cross-sectional view showing a step inmanufacturing the solid-state imaging device according to the embodiment2 of the present disclosure.

FIG. 10D illustrates a cross-sectional view showing a step inmanufacturing the solid-state imaging device according to the embodiment2 of the present disclosure.

FIG. 10E illustrates a cross-sectional view showing a step inmanufacturing the solid-state imaging device according to the embodiment2 of the present disclosure.

FIG. 11A illustrates a cross-sectional view showing a step inmanufacturing the solid-state imaging device according to the embodiment3 of the present disclosure.

FIG. 11B illustrates a cross-sectional view showing a step inmanufacturing the solid-state imaging device according to the embodiment3 of the present disclosure.

FIG. 11C illustrates a cross-sectional view showing a step inmanufacturing the solid-state imaging device according to the embodiment3 of the present disclosure.

FIG. 11D illustrates a cross-sectional view showing a step inmanufacturing the solid-state imaging device according to the embodiment3 of the present disclosure.

FIG. 11E illustrates a cross-sectional view showing a step inmanufacturing the solid-state imaging device according to the embodiment3 of the present disclosure.

FIG. 12 illustrates an exemplary configuration of an imaging apparatusaccording to any of the embodiments of the present disclosure.

FIG. 13 illustrates a circuit diagram of a pixel circuit in aconventional layered solid-state imaging device.

DESCRIPTION OF EMBODIMENTS

Hereinafter, the exemplary embodiments of the present disclosure aredescribed in detail with reference to the accompanying Drawings. Itshould be noted that the present disclosure is not limited to thefollowing embodiments. Furthermore, it is possible to advantageouslychange them as long as they are within the scope of the advantageouseffect of the present disclosure. It is also possible to combine withanother embodiment. Furthermore, any one of the embodiments to bedescribed below is for showing a mere example of the present disclosure.The numerical values, shapes, materials, constituent elements, thearrangement and connection of the constituent elements, steps, theprocessing order of the steps etc. shown in the following embodimentsare mere examples, and thus do not limit the present disclosure.Furthermore, among the constituent elements in the followingembodiments, constituent elements not recited in any of the independentclaims indicating the most generic concept of the present disclosure aredescribed as preferable constituent elements.

Embodiment 1

Concentrated implantation into a charge accumulation region, which isnecessary for the layered solid-state imaging device disclosed in PTL 1,enhances an electric field across a P-N junction, and this causesleakage current. In addition, the leakage current further increases whena sharp P-N junction is formed on an isolation region interface and asubstrate surface each having a high density of crystal defects.

Furthermore, like a conventional surface-illuminated solid-state imagingdevice, a method of accumulating, in an N-type charge accumulationregion, electrons which are generated in a silicon photodiode byphotoelectric conversion requires a high reset voltage to increase asaturation charge level, and the leakage current increases under a darkcondition. In contrast, a low reset voltage causes a problem of lackingthe number of electrons per pixel.

Furthermore, in the layered solid-state imaging device disclosed in PTL1, under an illumination condition, light leaks through a photoelectricconversion film to a substrate, and thus noise is generated when theleaked light is converted into electrons in the charge accumulationregion inside the substrate.

The present disclosure is intended to solve the problems of increasingnoise, increasing leakage current, and lacking the number of electronsper pixel as described above.

In a solid-state imaging device according to an embodiment 1 of thepresent disclosure, a semiconductor material is used for a contact plugfor electrically connecting to each other a photoelectric conversionfilm and a charge accumulation region which are essentials in a layeredsolid-state imaging device. As a result, an alloying reaction does notoccur at the contact interface between the charge accumulation regionand the contact plug. Thus, the solid-state imaging device can reducecrystal defects generated in a contact portion between the chargeaccumulation region and the contact plug, thereby decreasing noise.

First, a structure of the solid-state imaging device according to theembodiment 1 of the present disclosure is described. FIG. 1 illustratesa circuit diagram showing the structure of the solid-state imagingdevice according to the embodiment 1 of the present disclosure

As shown in FIG. 1, the solid-state imaging device according to theembodiment 1 of the present disclosure includes a semiconductorsubstrate 101, a plurality of pixels 11 arranged in a matrix above thesemiconductor substrate 101, a vertical scanning unit 13 (also referredto as a row scanning unit) for providing various timing signals to thepixels 11, a horizontal scanning unit 15 (also referred to as a columnscanning unit) for sequentially reading out signals from the pixels to ahorizontal output terminal 142, column signal lines 141 formed forrespective columns, and reset lines 126 provided for respective columnsto reset the pixels 11 to states under the dark condition. It should benoted that the “2×2” pixels 11 are shown in FIG. 1, but the number ofrows and columns may be determined advantageously.

Furthermore, each of the pixels 11 includes a photoelectric conversionunit 111, an amplification transistor 108 a having a gate connected tothe photoelectric conversion unit 111, a reset transistor 108 b having adrain connected to the photoelectric conversion unit 111, and aselection transistor 108 c connected in series to the amplificationtransistor 108 a.

The photoelectric conversion unit 111 includes a photoelectricconversion film for converting light into electrons, a pixel electrodeformed on a surface of the photoelectric conversion film facing to thesemiconductor substrate, and a transparent electrode formed on theopposite surface of the photoelectric conversion film. Thisphotoelectric conversion unit 111 is connected between a control line131 for the photoelectric conversion unit and the gate of theamplification transistor 108 a, i.e. the drain of the reset transistor108 b. The amplification transistor 108 a has a gate connected to thepixel electrode, and provides a signal voltage corresponding to avoltage of the pixel electrode to the column signal line 141 via theselection transistor 108 c. The reset transistor 108 b has a source andthe drain one of which is connected to the pixel electrode, and theother of which is connected to a corresponding reset line 126. Theselection transistor 108 c has a gate connected to the vertical scanningunit 13 via an address control line 121. The reset transistor 108 b hasa gate connected to the vertical scanning unit 13 via a reset controlline 123. The address control line 121 and the reset control line 123are provided for each of the rows.

In the embodiment, as an example, it is assumed that the resettransistor 108 b is an N-type MOS transistor, a reset pulse in a resetsignal provided to the gate of the reset transistor is a positive pulse(upward pulse), and the rear edge (back edge) of the reset pulse is afalling edge.

The control line 131 for the photoelectric conversion unit is shared byall the pixels. The column signal line 141 is provided for each of thecolumns, and connected to the horizontal scanning unit 15 via a columnsignal processing unit 21. The column signal processing unit 21 performsanalog-to-digital conversion, signal processing for suppressing noise asrepresented by correlated double sampling, and others.

Furthermore, when the reset transistor 108 b is in a conduction state,the voltage in the charge accumulation region 104 is 0 V or a positivevoltage of approximately 0 V.

FIG. 2 illustrates a timing chart showing the most basic imagingoperation of the solid-state imaging device according to the embodiment.In FIG. 2, SEL1 denotes a row selection signal for the first row. RST1denotes a row reset signal for the first row. SEL2 and RST2 differ fromSEL1 and RST1 only in that a corresponding row is different,respectively. One horizontal cycle is a period from a time point inwhich the row selection signal is enabled to a time point in which thenext row selection signal is enabled (from a rising edge of SEL1 to arising edge of SEL2), and a period required to read out the signalvoltages from the pixels in a single row. One vertical cycle is a periodrequired to read out the signal voltages from the pixels in all therows.

Furthermore, the amplification transistor 108 a, the selectiontransistor 108 c, and the reset transistor 108 b are formed on thesemiconductor substrate comprising silicon. The amplification transistor108 a has a gate electrode, the drain which is a diffusion layer, andthe source which is a diffusion layer. The selection transistor 108 chas a gate electrode, the drain which is a diffusion layer, and thesource which is a diffusion layer. The source of the amplificationtransistor 108 a and the drain of the selection transistor 108 c areformed in a common diffusion layer. The reset transistor 108 b has agate electrode, the drain which is a diffusion layer, and the sourcewhich is a diffusion layer. The amplification transistor 108 a and thereset transistor 108 b are separated by an element isolation region.

Furthermore, an insulating film is formed on the semiconductor substrate101 so as to cover the transistors. The photoelectric conversion unit111 is formed on the insulating film. The photoelectric conversion unit111 includes: a photoelectric conversion film comprising an organicmaterial, a material including a semiconductor as represented byamorphous silicon and germanium, or the like; the pixel electrode formedon the lower surface of the photoelectric conversion film; and thetransparent electrode formed on the upper surface of the photoelectricconversion film. The pixel electrode is connected, via a contact, to thegate electrode of the amplification transistor 108 a and the diffusionlayer which is the source of the reset transistor 108 b. The diffusionlayer connected to the pixel electrode acts as the charge accumulationregion.

As described above, the solid-state imaging device according to theembodiment includes the photoelectric conversion unit having a largelight absorption coefficient, and thus the quantum efficiency isexcellent.

Furthermore, in the solid-state imaging device according to theembodiment, the size of the charge accumulation region can be decreased,and thus it is possible to increase a conversion gain in terms of acircuit.

FIG. 3 illustrates a cross-sectional view of a structure including thecharge accumulation region 104 and the amplification transistor 108 aincluded in one pixel in the solid-state imaging device according to theembodiment 1 of the present disclosure.

As shown in FIG. 3, the pixel 11 includes: the element isolation region102 formed by providing an oxide film in the semiconductor substrate 101to isolate the transistors; a P-type impurity region 103 formed at aperiphery of the element isolation region 102 to suppress the leakagecurrent caused by an interface of the element isolation region 102; theN-type charge accumulation region 104 for accumulating the signalcharges from the photoelectric conversion film 114; a contact plug 107comprising polysilicon which includes N-type impurities at aconcentration higher than that in the charge accumulation region 104; animpurity diffusion layer 105 formed by diffusing the N-type impuritiesfrom the polysilicon; the gate electrode of the amplification transistor108 a formed on the semiconductor substrate 101 with a gate oxide film(not shown) provided therebetween; metal plugs 110 a to 110 d which arecontact plugs comprising a metal such as W, Cu or Al; insulator layers109 a to 109 d; a first electrode 113 (pixel electrode) isolated fromthe adjacent pixels 11 and connected to the charge accumulation region104 and the gate electrode of the amplification transistor 108 a; thephotoelectric conversion film 114 for generating the signal chargescorresponding to the amount of incident light; and a second electrode115 (transparent electrode) for applying to the photoelectric conversionfilm 114 a predetermined voltage necessary for the photoelectricconversion.

The photoelectric conversion film 114 generates charges according to theamount of light received. The holes of the generated charges areaccumulated in the charge accumulation region 104 via the firstelectrode 113 as the signal charges. On the other hand, the electronsare attracted to the second electrode 115. A voltage applied to the gateelectrode of the amplification transistor 108 a increases with theamount of the signal charges accumulated in the charge accumulationregion 104.

The amplification transistor 108 a outuputs a signal voltage based onthe signal charges accumulated in the charge accumulation region 104.The signal voltage output by the amplification transistor 108 a isprovided to the column signal line 141 by applying a predeterminedvoltage to the gate electrode of the selection transistor 108 c.

Furthermore, a voltage of the charge accumulation region 104 is set to areset voltage by applying a predetermined voltage to the gate electrodeof the reset transistor 108 b after signal read out.

In the embodiment, the charge accumulation region 104 and the metal plug110 a are connected to each other via the contact plug 107 comprisingpolysilicon doped to N⁺-type conductivity. After this contact plug 107is formed, the impurity diffusion layer 105 is formed by annealing. Thisimpurity diffusion layer 105 can reduce the contact resistance betweenthe contact plug 107 and the charge accumulation region 104.Furthermore, due to the formation of the impurity diffusion layer 105,the charge accumulation region 104 is not required to be N⁺-typeconductivity, and thus the charge accumulation region 104 is formed asN⁻-type conductivity. Furthermore, the P-type impurity region 103 isformed at the periphery of the element isolation region 102 to preventthe leakage current generated at the interface of the element isolationregion 102. It should be noted that the impurity diffusion layer 105need not be always formed as long as the contact resistance between thecontact plug 107 and the charge accumulation region 104 is within anallowable range.

Here, FIG. 4 shows an electric field intensity at the P-N junction whenan impurity concentration in the charge accumulation region 104 isplotted along the horizontal axis. In FIG. 4, it is assumed that theP-type impurity region 103 has a concentration of 10 ¹⁸ cm⁻³. As shownin FIG. 4, the electric field intensity can be reduced with decreasingthe impurity concentration in the charge accumulation region 104.Furthermore, when a strong electric field is applied to the P-Njunction, the leakage current caused by TAT (Trap Assisted Tunneling)current is dominant. In this model, the leakage current increases withincreasing the electric field intensity. Accordingly, it is found thatthe reducing of the impurity concentration in the charge accumulationregion 104 is effective for the decrease in the leakage current.

In view of the forgoing structure, the charge accumulation region 104 isisolated by the P-type layer from the element isolation region 102comprising an oxide film. Furthermore, the impurity diffusion layer 105is minimized which includes impurities at a high concentration and isnecessary to reduce the contact resistance. Accordingly, it is possibleto suppress the leakage current generated at the periphery of the chargeaccumulation region 104 as much as possible. Furthermore, this structurehas an advantage in miniaturization.

Furthermore, in the embodiment, when the holes are read out, the chargeaccumulation region 104 which is an N-type impurity region is used. Withthis, under the illumination condition, a voltage applied between thecharge accumulation region 104 and the semiconductor substrate 101 is areverse-bias voltage. Accordingly, this structure is advantageous inthat (i) the leakage current increases less than that of a method ofaccumulating electrons in the N-type impurity region and (ii) themaximum number of electrons per pixel increases.

Furthermore, in the embodiment, a thick polysilicon plug is providedbetween the metal plug 110 a and the contact plug 107 comprisingpolysilicon. This interlayer has an effect to prevent light passingthrough the photoelectric conversion film 114 from directly entering thecharge accumulation region 104. Thus, it is possible to suppress thenoise. Furthermore, the thick interlayer can be also used to preventmisalignment between the thin plugs. However, the thick polysilicon plugneed not be always formed when the coupling capacity between theadjacent pixels is increased by the thick polysilicon plug and thuscolor cross-talk is also increased.

Furthermore, in the embodiment, although the polysilicon is used as amaterial of the contact plug 107, a material including polycrystallinesilicon, Ge, or GaAs may be used instead of the polysilicon.

As described above, the solid-state imaging device according to theembodiment 1 of the present disclosure is a solid-state imaging deviceincluding a plurality of pixels 11 arranged in a matrix, the solid-stateimaging device including: a semiconductor substrate 101; a firstelectrode 113 formed above the semiconductor substrate 101 for each ofthe pixels 11, and electrically isolated from an adjacent one of thepixels 11; a photoelectric conversion film 114 formed on the firstelectrode 113, the photoelectric conversion film converting light intosignal charges; a second electrode 115 formed on the photoelectricconversion film 114; a charge accumulation region 104 formed in thesemiconductor substrate 101 for each of the pixels 11, and electricallyconnected to the first electrode 113 in the pixel 11, the chargeaccumulation region accumulating the signal charges converted from thelight by the photoelectric conversion film 114; a reset gate electrodeformed for each of the pixels 11, the reset gate electrode resetting thecharge accumulation region; an amplification transistor 108 a formed foreach of the pixels 11, the amplification transistor amplifying thesignal charges accumulated in the charge accumulation region 104 in thepixel 11; and a contact plug 107 formed, for each of the pixels 11, indirect contact with the charge accumulation region 104 and comprising asemiconductor material, the contact plug being for electricallyconnecting to each other the first electrode 113 and the chargeaccumulation region 104 in the pixel 11.

In the solid-state imaging device according to the embodiment 1 of thepresent disclosure, a semiconductor material is used for the contactplug for electrically connecting to each other the photoelectricconversion film 114 and the charge accumulation region 104 which areessentials in the layered solid-state imaging device. With this, thealloying reaction does not occur at the contact interface between thecharge accumulation region 104 and the contact plug 107. Thus, thesolid-state imaging device can reduce the crystal defects generated inthe contact portion between the charge accumulation region 104 and thecontact plug 107, thereby decreasing the noise.

Furthermore, the charge accumulation region 104 has a conductivity typeidentical to a conductivity type of the semiconductor material includedin the contact plug 107.

With this, there is no difference in potential between the chargeaccumulation region 104 and the contact plug 107, and thus the contactresistance between the charge accumulation region 104 and the contactplug 107 is reduced.

Furthermore, a concentration of impurities determining the conductivitytype of the semiconductor material included in the contact plug 107 ishigher than a concentration of impurities determining the conductivitytype of the charge accumulation region 104

With this, it is possible to decrease the potential barrier existingbetween the charge accumulation region 104 and the contact plug 107, andthus the contact resistance between the charge accumulation region 104and the contact plug 107 can be further reduced. Furthermore, thecontact resistance can be also reduced even when the impurityconcentration in the charge accumulation region 104 is lowered, and thusthe effect of a reduction in the leakage current due to a loweredconcentration in the charge accumulation region 104 can be achieved.

Furthermore, the charge accumulation region 104 includes an impuritydiffusion layer 105 in direct contact with the contact plug 107, and aconcentration of impurities determining a conductivity type of theimpurity diffusion layer 105 is higher than a concentration ofimpurities determining a conductivity type of a region, other than theimpurity diffusion layer 105, included in the charge accumulation region104.

With this, the contact resistance between the contact plug 107 and thecharge accumulation region 104 can be reduced.

Furthermore, the signal charges have polarities opposite to polaritiesof majority carriers determining a conductivity type of the chargeaccumulation region 104.

With this, it is possible to decrease the reset voltage between chargeaccumulation region 104 and the substrate 101 while maintaining the highsaturation signal level, and thus the leakage current can be suppressedunder the dark condition.

On the other hand, contrary to such a structure, when the signal chargeshave polarities equivalent to polarities of carriers determining aconductivity type of the charge accumulation region 104, it is needed tomake a choice between the two: the leakage current is traded off againstthe saturation signal level to keep it high by setting the reset voltageto a high reverse-bias voltage; and the saturation signal level istraded off against the leakage current by setting the reset voltage to alow voltage to suppress the leakage current.

The following describes in detail an exemplary structure in which thecharge accumulation region 104 is N-type conductivity. In view of theforegoing structure, under the dark condition, the voltage of the chargeaccumulation region 104 is 0 V or a positive voltage of approximately 0V. FIG. 5A illustrates a schematic view showing a band structure along aline A0-A1 under the dark condition when the charge accumulation region104 is formed as the N-type conductivity. In addition, FIG. 5Billustrates a graph showing a charge profile along the line A0-A1. Theline A0-A1 refers to a line through the contact plug 107 and the chargeaccumulation region 104 shown in FIG. 3.

In this case, depletion layer charges 118 are formed in the chargeaccumulation region 104. Under the dark condition, a difference inpotential between the charge accumulation region 104 and thesemiconductor substrate 101 is 0 V or approximately 0 V, and thus it ispossible to reduce the leakage current between the charge accumulationregion 104 and the semiconductor substrate 101. In contrast, under theillumination condition, the charge accumulation region 104 is chargedpositively.

FIG. 5C illustrates a schematic view showing a band structure along theline A0-A1 under the illumination condition when the charge accumulationregion 104 is formed as the N-type conductivity. FIG. 5D illustrates agraph showing a charge profile along the line A0-A1.

The signal charges have polarities opposite to polarities of majoritycarriers in the charge accumulation region 104, and thus the chargeprofile in FIG. 5D shows a shape in which signal charges 119 formed asspace charges is added to the depletion layer charges 118 under the darkcondition. As a result, under the illumination condition, a reverse-biasvoltage is applied between the charge accumulation region 104 and thesemiconductor substrate 101. Accordingly, it is possible to apply avoltage to the charge accumulation region 104 until the voltage reachesthe reverse breakdown voltage of the P-N junction, and thus a highsaturation signal level can be obtained.

Furthermore, the solid-state imaging device further including: anelement isolation region 102 formed in the semiconductor substrate 101and comprising an insulator, the element isolation region isolating thecharge accumulation region 104 from a charge accumulation region 104 inan adjacent one of the pixels and a transistor region in which theamplification transistor 108 a is formed; and a P-type impurity region103 formed between the element isolation region 102 and the chargeaccumulation region 104 in the semiconductor substrate 101, and having aconductivity type opposite to a conductivity type of the chargeaccumulation region 104, in which the P-type impurity region 103 has aconcentration of impurities that is higher than a concentration ofimpurities in the charge accumulation region 104 and lower than aconcentration of impurities in the contact plug 107.

The element isolation region 102 surrounds the amplification transistor108 a and the selection transistor 108 c. The element isolation region102 also surrounds the charge accumulation region 104 and the resettransistor 108 b. Thus, the charge accumulation region 104 is isolatedfrom the adjacent pixels.

Such a structure ensures the insulation of the element isolation region102 and further makes it possible to suppress the leakage currentgenerated in the element isolation region 102.

Furthermore, the contact plug 107 includes silicon or germanium.

With this, it is possible to suppress a dark current because a materialappropriate to a silicon process is selected for a material of thecontact plug 107 used to eliminate interface defects at the surface ofthe charge accumulation region 104.

Embodiment 2

In an embodiment 2 according to the present disclosure, a variation ofthe embodiment 1 is described.

FIG. 6 illustrates a cross-sectional view of a structure including acharge accumulation region 104 and an amplification transistor 108 aincluded in one pixel in the solid-state imaging device according to theembodiment 2 of the present disclosure. It should be noted that anelement identical to that in FIG. 3 is numbered the same.

In addition, the following mainly describes differences from theembodiment 1, and the detailed description of the same is omitted.

A structure shown in FIG. 6 includes a P-type impurity region 116 formedto isolate transistor regions from each other, instead of the elementisolation region 102 and the P-type impurity region 103 in the structureshown in FIG. 3. Furthermore, a shape of a contact plug 107 and a sizeof the charge accumulation region 104 are different from those in FIG.3.

The contact plug 107 comprising polysilicon according to the embodimenthas a top width greater than a bottom width thereof. With this, it ispossible to prevent a bad connection caused by misalignment which occurswhen a metal plug 110 a is formed on the contact plug 107.

Furthermore, the charge accumulation region 104 according to theembodiment is minimized. Such a charge accumulation region 104 is formedby implanting impurities through a connect hole before the formation ofthe contact plug 107. In this method, the charge accumulation region 104can be formed at a maximum distance from the P-type impurity region 116.However, in order to make a reset transistor 108 b operable, a method ofshortening a distance between the contact plug 107 and the resettransistor 108 b, or a method of forming the charge accumulation regionby implanting impurities using another mask from a forming position ofthe contact plug to the bottom of the gate electrode of the resettransistor may be used.

With this, the solid-state imaging device according to the embodimentcan achieve the same effect as the foregoing embodiment 1.

Furthermore, the contact plug 107 has the bottom width smaller than thetop width thereof.

This prevents leakage light passing through the photoelectric conversionfilm 114 from entering the charge accumulation region 104 under anillumination condition, thereby suppressing noise. Furthermore, it ispossible to prevent the bad connections.

FIG. 7 illustrates a pixel circuit configuration according to theembodiment 2. In the embodiment, for the sake of the miniaturization,the gate electrode of the reset transistor 108 b and the gate electrodeof the selection transistor 108 c are formed with a common gateelectrode and wiring. Furthermore, in the pixel circuit shown in FIG. 7,in order to drive the reset transistor 108 b and the selectiontransistor 108 c independently, the reset transistor 108 b has athreshold voltage higher than that of the selection transistor 108 c.Accordingly, it is possible to turn OFF the reset transistor 108 b andturn ON the selection transistor 108 c at a common gate voltage.

Embodiment 3

In an embodiment 3 according to the present disclosure, a variation ofthe embodiment 1 is described.

FIG. 8 illustrates a cross-sectional view of a structure including acharge accumulation region 104 and an amplification transistor 108 aincluded in one pixel in the solid-state imaging device according to theembodiment 3. It should be noted that an element identical to that inFIG. 3 is numbered the same.

In a structure shown in FIG. 8, a P-type impurity region 106, a metalplug 110 e, and an N-type impurity region 117 are added to the elementsshown in FIG. 3.

Furthermore, in the embodiment, the metal plug 110 e formed on thecontact plug 107 comprising polysilicon has a width greater than that ofthe contact plug 107. With this, it is possible to prevent misalignmentbetween them.

As described above, the solid-state imaging device according to theembodiment 3 further includes: a P-type impurity region 106 formed in aregion not in contact with the contact plug at a surface of the chargeaccumulation region 104, and having a conductivity type opposite to aconductivity type of the charge accumulation region 104.

With this, in the charge accumulation region 104, the effect of theleakage current generated at the surface of a semiconductor substrate101 on the charge accumulation region 104 is suppressed.

Here, when the P-type impurity region 106 makes it difficult toelectrically connect the contact plug 107 and the charge accumulationregion 104 to each other, supplementarily, an N-type impurity region 117may be additionally formed by implanting impurity through a groove forforming the contact plug.

It should be noted that the other effects are the same as in theembodiment 1.

Manufacturing Method According to Embodiment 1

Referring to cross-sectional views as shown in FIG. 9A to FIG. 9E, amethod of manufacturing the solid-state imaging device according to theforegoing embodiment 1 is described generally below.

First, as shown in FIG. 9A, using a general method of forming a layeredsolid-state imaging device, the element isolation region 102, the P-typeimpurity region 103 at the periphery of the element isolation region102, and MOS transistors (the amplification transistor 108 a, the resettransistor 108 b, and the selection transistor 108 c) each having thegate electrode are formed on the semiconductor substrate 101. At thesame time as the formation of the MOS transistors, transistors includedin a peripheral circuit for performing signal processing are alsoformed. Moreover, the N⁻-type charge accumulation region 104 is formedby implanting ions.

Subsequently, as shown in FIG. 9B, the insulator layer 109 a isdeposited using a sputtering method or a CVD method. After this, acontact hole 107 a is formed in the insulator layer 109 a at a positionwhere the wiring and the contact plug 107 comprising polysilicon havingN⁺-type impurities are to be formed.

Subsequently, as shown in FIG. 9C, the highly concentrated polysiliconhaving N-type impurities is deposited using the CVD method or thespattering method.

Subsequently, as shown in FIG. 9D, a part of the polysilicon 107 b isremoved through an etching process to leave an interlayer having aminimum size required to be used as light shielding of the chargeaccumulation region 104 or to prevent misalignment. Then, afterdepositing an oxide film, the impurity diffusion layer 105 is formedthrough an annealing process at a high temperature of 700 to 900degrees.

Subsequently, as shown in FIG. 9E, the metal plug 110 a and the wiring112 a are formed using a general method.

Subsequently, the structure shown in FIG. 3 is achieved by forming themetal plugs 110 b to 110 d, the wirings 112 b to 112 c, the insulatorlayers 109 b to 109 c, the first electrode 113, the photoelectricconversion film 114, the second electrode 115, a protection film (notshown), a color filter (not shown), and a lens (not shown). It should benoted that these manufacturing steps are the same as those of theconventional layered solid-state imaging device, and thus the detaileddescription is omitted here.

Manufacturing Method According to Embodiment 2

Referring to cross-sectional views as shown in FIG. 10A to FIG. 10E, amethod of manufacturing the solid-state imaging device according to theforegoing embodiment 2 is described generally below.

First, as shown in FIG. 10A, using a general method of forming a layeredsolid-state imaging device, the P-type impurity region 116 and the MOStransistors (the amplification transistor 108 a, the reset transistor108 b, and the selection transistor 108 c) are formed on thesemiconductor substrate 101. At the same time, a peripheral circuit forperforming signal processing is also formed.

Subsequently, as shown in FIG. 10B, the insulator layer 109 a isdeposited using a sputtering method or a CVD method. After this, acontact hole 107 a is formed in the insulator layer 109 a at a positionwhere the wiring and the contact plug 107 comprising polysilicon havingN⁺-type impurities are to be formed. After this, the charge accumulationregion 104 is formed by implanting N-type impurities through the contacthole 107 a.

Subsequently, as shown in FIG. 10C, the polysilicon 107 b having N⁺-typeimpurities is deposited using the CVD method or the spattering method.

Subsequently, as shown in FIG. 10D, a part of the polysilicon 107 b isremoved through a dry-etching process to leave an interlayer having aminimum size required to be used as light shielding of the chargeaccumulation region 104 or to prevent misalignment. Then, afterdepositing an oxide film, the impurity diffusion layer 105 is formedthrough an annealing process at a high temperature of 700 to 900degrees.

Subsequently, as shown in FIG. 10E, the metal plug 110 a and the wiring112 a are formed using a general method.

Subsequently, the structure shown in FIG. 6 is achieved by forming themetal plugs 110 b to 110 d, the wirings 112 b to 112 c, the insulatorlayers 109 b to 109 c, the first electrode 113, the photoelectricconversion film 114, the second electrode 115, a protection film (notshown), a color filter (not shown), and a lens (not shown). It should benoted that these manufacturing steps are the same as those of theconventional layered solid-state imaging device, and thus the detaileddescription is omitted here.

Manufacturing Method According to Embodiment 3

Referring to cross-sectional views as shown in FIG. 11A to FIG. 11E, amethod of manufacturing the solid-state imaging device according to theforegoing embodiment 3 is described generally below.

First, as shown in FIG. 11A, using a general method of forming a layeredsolid-state imaging device, the P-type impurity region 116 and the MOStransistors (the amplification transistor 108 a, the reset transistor108 b, and the selection transistor 108 c) are formed on thesemiconductor substrate 101. At the same time, a peripheral circuit forperforming signal processing is also formed. In this step, the P-typeimpurity region 106 is formed on the charge accumulation region 104.

Subsequently, as shown in FIG. 11B, the insulator layer 109 a isdeposited using a sputtering method or a CVD method. After this, acontact hole 107 a is formed in the insulator layer 109 a at a positionwhere the wiring and the contact plug 107 comprising polysilicon havingN⁺-type impurities are to be formed. After this, the N-type impurityregion 117 is formed by implanting N-type impurities through the contacthole 107 a as needed.

Subsequently, as shown in FIG. 11C, the polysilicon 107 b having N⁺-typeimpurities is deposited using the CVD method or the spattering method.

Subsequently, as shown in FIG. 11D, a part of polysilicon is removed bypolishing it through a CMP process to leave the contact plug 107. Then,after depositing an oxide film, the impurity diffusion layer 105 isformed through an annealing process at a high temperature of 700 to 900degrees.

Subsequently, as shown in FIG. 11E, the metal plugs 110 a and 110 e andthe wiring 112 a are formed using a general method. At this step, inorder to prevent misalignment with the contact plug 107, the metal plug110 e is formed so as to have a width greater than that of the metalplug 110 a.

Subsequently, the structure shown in FIG. 8 is achieved by forming themetal plugs 110 b to 110 d, the wirings 112 b to 112 c, the insulatorlayers 109 b to 109 c, the first electrode 113, the photoelectricconversion film 114, the second electrode 115, a protection film (notshown), a color filter (not shown), and a lens (not shown). It should benoted that these manufacturing steps are the same as those of theconventional layered solid-state imaging device, and thus the detaileddescription is omitted here.

Imaging Apparatus Including Solid-State Imaging Device According toEmbodiment

The following describes an imaging apparatus (camera) including asolid-state imaging device described in any of the foregoing embodiments1 to 3.

FIG. 12 illustrates an overall configuration of the imaging apparatus200 including the solid-state imaging device according to theembodiment. The imaging apparatus 200 including the solid-state imagingdevice according to the embodiment includes a lens 201, a solid-stateimaging device 206, a signal processing circuit 207, and an outputinterface 209.

The solid-state imaging device 206 is a solid-state imaging devicedescribed in any of the foregoing embodiments 1 to 3. A pixel array 202has the foregoing pixels 11 arranged in a matrix. A row selectioncircuit 203 and a column selection circuit 204 corresponds to thevertical scanning unit 13 and the horizontal scanning unit 15 shown inFIG. 1, respectively.

The lens 201 forms an image of an object on the pixel array 202. Thesignals obtained by the pixel array 202 are sequentially transmitted tothe signal processing circuit 207 via the row selection circuit 203, thecolumn selection circuit 204, and a read-out circuit 205. The signalprocessing circuit 207 performs signal processing on the receivedsignal, and outputs the resulting image signal to the output interface209 including a display and a memory.

The imaging apparatus and the solid-state imaging device according tothe embodiment of the present disclosure are described above, but thepresent disclosure is not limited to the embodiment.

For example, the semiconductor substrate 101 in the foregoingdescription may be replaced with a well formed on the semiconductorsubstrate 101.

The solid-state imaging device according to the embodiment is typicallyimplemented as a large-scale integration (LSI) circuit, which is anintegrated circuit. These may be integrated into separate chips, or someor all of them may be integrated into a single chip.

The integration may be achieved, not only as a LSI, but also as adedicated circuit or a general purpose processor. Also applicable is afield programmable gate array (FPGA), which allows post-manufactureprogramming, or a reconfigurable processor LSI, which allowspost-manufacture reconfiguration of connection and setting of circuitcells therein.

In the foregoing cross-sectional views, the corner and the side of eachconstituent element are linearly drawn, but the constituent elementsincluding round corners and curved sides in terms of the manufacturingprocess are also included in the present disclosure.

At least some of functions of the solid-state imaging device, theimaging apparatus, and modifications thereof according to any of theforegoing embodiments 1 to 3 may be combined.

All the figures used above are provided for purposes of illustration ofthe present disclosure, and the present disclosure is not limited tothese figures. The logic levels represented as High/Low levels or theswitching sates represented as ON/OFF are provided for purposes ofillustration of the present disclosure, and an equivalent result can beobtained by a different combination of the logic levels or the switchingstates. The N-type and P-type of a transistor, an impurity region, andothers are provided for purposes of illustration of the presentdisclosure, and an equivalent result can be obtained by reversing thesetypes. All the materials of the foregoing constituent elements areexamples for illustrating the present disclosure, and the presentdisclosure is not limited to these materials. The relation of connectionbetween the constituent elements is an example for illustrating thepresent disclosure, and the relation of connection for achieving thefunctions of the present disclosure is not limited to this.

The partition of function blocks in the block diagram is an example, andthe function blocks may be integrated into a single function block, asingle function block may be divided into some blocks, and a part of thefunction may be transferred to another function block. A single hardwareor software may process the functions of the function blocks havingsimilar functions, in parallel or in a time division method.

In the foregoing description, MOS transistors are used as examples, butother transistors are possible.

Various modifications to the embodiments that can be conceived by thoseskilled in the art which are within the teachings of the presentdisclosure may be included in the scope of the present disclosure.

INDUSTRIAL APPLICABILITY

The present disclosure can be applied to a solid-state imaging device.The present disclosure also can be applied to an imaging apparatusincluding a solid-state imaging device, such as a digital still camera,a digital video camera, a mobile camera, a monitoring camera.

1. A solid-state imaging device including a plurality of pixels arrangedin a matrix, the solid-state imaging device comprising: a semiconductorsubstrate; a first electrode formed above the semiconductor substratefor each of the pixels, and electrically isolated from an adjacent oneof the pixels, a photoelectric conversion film formed on the firstelectrode, the photoelectric conversion film converting light intosignal charges; a second electrode formed on the photoelectricconversion film; a charge accumulation region formed in thesemiconductor substrate for each of the pixels, and electricallyconnected to the first electrode in the pixel, the charge accumulationregion accumulating the signal charges converted from the light by thephotoelectric conversion film; a reset gate electrode formed for each ofthe pixels, the reset gate electrode resetting the charge accumulationregion; an amplification transistor formed for each of the pixels, theamplification transistor amplifying the signal charges accumulated inthe charge accumulation region in the pixel; and a contact plug formed,for each of the pixels, in direct contact with the charge accumulationregion and comprising a semiconductor material, the contact plug beingfor electrically connecting to each other the first electrode and thecharge accumulation region in the pixel.
 2. The solid-state imagingdevice according to claim 1, wherein the charge accumulation region hasa conductivity type identical to a conductivity type of thesemiconductor material included in the contact plug.
 3. The solid-stateimaging device according to claim 2, wherein a concentration ofimpurities determining the conductivity type of the semiconductormaterial is higher than a concentration of impurities determining theconductivity type of the charge accumulation region.
 4. The solid-stateimaging device according to claim 3, wherein the charge accumulationregion includes an impurity diffusion layer in direct contact with thecontact plug, and a concentration of impurities determining aconductivity type of the impurity diffusion layer is higher than aconcentration of impurities determining a conductivity type of a region,other than the impurity diffusion layer, included in the chargeaccumulation region.
 5. The solid-state imaging device according toclaim 1, wherein the signal charges have polarities opposite topolarities of majority carriers determining a conductivity type of thecharge accumulation region.
 6. The solid-state imaging device accordingto claim 1, further comprising: an element isolation region formed inthe semiconductor substrate and comprising an insulator, the elementisolation region isolating the charge accumulation region from a chargeaccumulation region in an adjacent one of the pixels and a transistorregion in which the amplification transistor is formed; and an impurityregion formed between the element isolation region and the chargeaccumulation region in the semiconductor substrate, and having aconductivity type opposite to a conductivity type of the chargeaccumulation region, wherein the impurity region has a concentration ofimpurities that is higher than a concentration of impurities in thecharge accumulation region and lower than a concentration of impuritiesin the contact plug.
 7. The solid-state imaging device according toclaim 1, further comprising an impurity region formed in a region not incontact with the contact plug at a surface of the charge accumulationregion, and having a conductivity type opposite to a conductivity typeof the charge accumulation region.
 8. The solid-state imaging deviceaccording to claim 1, wherein the contact plug has a bottom widthsmaller than a top width of the contact plug.
 9. The solid-state imagingdevice according to claim 1, wherein the contact plug includes siliconor germanium.